Heart Rate Detection from Impedance Plethysmography Based on Concealed Capacitive Electrodes

 Physiological monitoring outside clinical environments and medical offices can contribute to people’s wellness, particularly if that monitoring does not disturb their daily activities, and can also help in life-style improvement. We propose a novel non-conscious method to monitor the heart rate which is based on detecting blood volume changes in the thigh through electrical impedance plethysmography while the subject is seated. Four conductive strip electrodes are concealed underneath seat’s upholstery, hence working as capacitive electrodes. A 50 kHz current is injected trough two outer electrodes and the changes in the drop in voltage between the inner electrode pair are detected. Tests performed with a system designed in our laboratory show that the recorded signal has peaks that unmistakably coincide with the heart rate. The waveform depends on the relative position of both pairs of electrodes, and the sensitivity to impedance changes depends on the position and area of the voltage-detecting electrodes with respect to those of the current injection electrodes. The amplitude and waveform of the recorded signal are good enough for the heart rate to be straightforwardly obtained by a slope-detection-based algorithm.